The Massachusetts senator is unquestionably the front-runner for the Democratic presidential nomination, which made her the focal point of attacks from her rivals in Tuesday’s Democratic debate. Former Vice President Joe Biden had an easier time of it, but this was not good news for him. At various points in the three-hour marathon, he faded away. His rivals no longer see him as the person to beat.

Beneath the jostling for position lay an important policy subtext. Warren was plainly far more comfortable defending her broader economic positions, particularly her proposed wealth tax, than she was in standing up for Medicare for All. The wealth on tax goes to the heart of her candidacy’s purposes. Medicare for All appears to be a position she adopted — somewhat belatedly — to fend off Sen. Bernie Sanders and attacks from her left. Her continued success depends on how she handles this tension.

Reps. Alexandria Ocasio-Cortez, D-N.Y., and Ilhan Omar, D-Minn., underscored the ongoing significance of the battle for the party's progressive wing by endorsing Sanders, who strengthened his hand with a solid performance. Their decisions defined the difficult political terrain Warren faces. Her rise depended on her ability, simultaneously, to pull left-wing Democrats away from Sanders while also gaining new support from more moderate progressives. She made it look effortless. It will be effortless no longer.

The debate also marked a reconfiguration at the other end of the party. While Biden remains near (and, in some polls, still at) the top of the field, South Bend Mayor Pete Buttigieg and Sen. Amy Klobuchar, D-Minn., sense it's now-or-never for them to replace Biden as the moderate alternative to Warren.

Biden had his moments on Tuesday, and he was especially passionate when the conversation turned to foreign policy and President Trump's decision to withdraw American troops from Syria, abandoning the United States' Kurdish allies. "This is shameful, shameful what this man has done," Biden declared.

Nonetheless, it was Buttigieg especially, but also Klobuchar, who dominated the discussion on the party's center-left. Up to now, Buttigieg has prospered as a refreshing new voice in the party, but struggled to break out of single digits. He was earning more acclaim than support. On Tuesday, he played to win.

He was scalding in challenging Warren's refusal to say whether she would have to raise middle-class taxes to pay for Medicare for All. "Your signature, Senator, is to have a plan for everything. Except this," Buttigieg said. "No plan has been laid out to explain how a multi-trillion-dollar hole in this Medicare for all plan that Senator Warren is putting forward is supposed to get filled in."

Buttigieg shrewdly defined his space: bolder than Biden ("bold" was one of his favorite words), but with more practical, do-able ideas than Warren, and more of an "outsider" than the Congressional veterans on the stage. Klobuchar was also forceful — particularly in discussing the opioid crisis — blending moderation with family anecdotes that brought home her Minnesota roots. Still, both Midwesterners need Biden to weaken substantially and rather quickly, and the former vice president may be doing well enough to keep that from happening.

But will Klobuchar and Buttigieg slow Warren's rise? Perhaps surprisingly, the moderates and Sanders share an interest in pointing out that Warren has not always been as enthusiastic about Medicare for All as she is now. She was careful in a New York Times interview earlier this year to endorse it, but she stressed the goal of "moving us to a place where everybody is covered at the lowest possible cost" and stressed "there are a lot of different ways to get there. 'Medicare for All' has a lot of different paths for how we get there." One can imagine her returning to this more circumspect position in a general election campaign, made easier by her choice to avoid issuing a comprehensive health care proposal of her own.

On the other hand, Warren was unabashed and effective defending her wealth tax — "Taxing income is not going to get you where you need to be the way taxing wealth does" — and she can already fairly claim that she has fundamentally altered the national conversation on taxing large fortunes.

"I have a plan for that" has been Warren's iconic soundbite. To this point, her strategy has worked brilliantly. Going forward, she will need to confront challenges from both her left and her right. She needs a plan for that, too.